Wireless communication devices typically include an antenna to facilitate the transmission and reception of radio frequency (RF) electromagnetic signals. An antenna has a characteristic impedance, or opposition to the flow of electrical energy, based on its design for a given set of certain operating conditions. In order to maximize the transfer of power to and from the antenna, it is desirable to match the impedance of the antenna to the impedance of the circuit that is driving the antenna. Impedance matching networks are well known tools for matching the impedance of a load, such as an antenna, to a source, such as a radio transceiver.
However, the actual impedance presented by an antenna can change as the antenna is being used. For example, the impedance of the antenna can change when it is held in different orientations, or when it is held near an external object, such as a user's head or hand.
The performance of the antenna can also be affected by the operating frequency of the antenna and the mode of operation of the antenna. Furthermore, many wireless terminals can be used in various configurations. For example, wireless mobile telephones may have designs, such as clamshell (flip phone), jackknife and slider configurations that typically include two housings coupled together by a hinge or slide. Such configurations may permit the mobile telephone to operate in an open position or a closed position. Typically, the antenna of the device is oriented in one direction when the device is in the open position and in a different direction when the device is in the closed position.
Similarly, mobile telephones can be used in a handheld talk position in which the telephone is physically held near the user's head, or a handsfree mode in which the mobile telephone is held near the user's body or is even not held by the user at all. Since antennas for wireless devices typically have limited bandwidth and can only be optimized for one use mode (such as for a handheld talk position), it may be difficult to maintain an effective impedance match for the antenna when, for example, the user places a finger over the antenna.
Changing the antenna matching in response to changes in the mode of operation of the device is usually called antenna tuning. Presently, antenna tuning involves switching an impedance matching circuit between two different impedances. Switching the impedance match may be used to change the resonant frequency of the antenna in broad band applications or to compensate the antenna match depending on whether the device is being used in an open or closed position. However, such an approach may not provide adequate flexibility in tuning the antenna match to accommodate different types of changes in the operating environment of the device. Component and filter manufacturers are developing filter banks, component banks and even tunable components, such as micro electrical-mechanical (MEMS) devices, that are aimed at improving the process of antenna tuning.